Railroads is a good example--and sometimes I'll throw out examples, you're not responsible for them, it's just kind of hopefully interesting for you to think about.

铁路就是很好的例子,有时我会举一些例子,你们可以不去记他们,我这么做是希望你们可以去思考

耶鲁公开课 - 1871年后的法国课程节选